


Blackened Tide

by Capns_scrolls



Category: Voltron: Legendary Defender
Genre: Heavily based on H2O: Just Add Water, I hope I do ok, M/M, Magic, Mer!Lance, Rivals to Friends to Lovers, Rivals to Lovers, Slow Burn, This Is Incredibly Self-Indulgent, Water Magic, and my first kl fic, mer!Keith
Language: English
Status: In-Progress
Published: 2020-05-15
Updated: 2020-05-15
Packaged: 2021-03-02 22:01:14
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,492
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/24193969
Author URL: https://archiveofourown.org/users/Capns_scrolls/pseuds/Capns_scrolls
Summary: One night in the mouth of a volcano was all it took to change Keith's life forever.After two years of mastering his gift, he must do more than protect himself. He also must protect others like him, namely the stupid boy he's fallen for.ORAn H2O: Just Add Water AU because I suck at summaries.
Relationships: Keith/Lance (Voltron)
Comments: 5
Kudos: 42





	Blackened Tide

**Author's Note:**

> It's mermay and I have no self control. That's it, that's the only reason this fic exists.

At fourteen, everyone thinks they’re ready for the world. Invincible, able to take on anything and think for themselves. Keith was certainly not an exception.

For as long as he could remember, Keith had always wanted to be an explorer. Explorers didn’t show fear. No matter how steep the incline or how far down the chasm dropped. Whatever fear Keith did have was dropped into that pit, and he’d sprint a great distance before it hit the bottom.

He felt that emotion once, and never wanted to feel that emotion again. So when his brother had warned him to stay away from that volcano, he didn’t listen.

That same night, Keith prepared. He had his goggles around his neck, waterproof LED flashlight in his pocket, even his sneakers and swim trunks were on as he got under the covers of his bed and pretended to sleep. 

He waited for Shiro’s soft snores to fill the hall before making his way out his bedroom window with silent movements, closing it with the softest sound he could manage.

His bike took him as far as it could on the asphalt roads and gravel trail leading to the volcano base. When he arrived, he found a beach. One he’d never seen or heard of. It was small, probably no more than 20 meters in width. Black sand covered the expanse of the space that abruptly ended at the foot of the obsidian cliffside. The smoothest, cleanest sand that Keith has ever seen or felt.

From the looks of it, most people avoided the beach. Signs were posted around the entrance, warning about the dangerous animals in the water, but it didn’t seem like the water was unfit to swim in quality-wise. Keith shined his flashlight out to the calm water. Buoys with cables connecting them bobbed in the water. No large water vehicles could get past them, except for kayakers and paddle boarders, if they were brave enough to go into the bay.

Well, if those imaginary kayakers weren’t, Keith certainly was. He didn’t settle for getting his feet wet, he pressed his goggles against his eyes and kicked his shoes off in the direction of his toppled bike and ran into the water. He dove down towards the sand floor and let his flashlight lead the way. 

The only animals he saw were slender sharks no longer than the length of his hand and pearly, jellyfish here and there. The coral reefs had urchins on them, but they were easily avoidable. From what Keith saw, there was nothing dangerous here.

He swam out to one of the cables, breaking the surface and resting his arms and upper back on it to observe the lights of his town.

Keith loved where he lived. Every night was a colorful light show that stretched for miles and reflected like jewels in the water. The edge of the city granted access to the clearest water for swimming. However, if there was one thing he’d change about it, it would be to have the city go completely dark in the night. He could see the stars, but they were faint glimpses that dotted the sky on the nights where the sky was the most visible. He wanted the view where he could see the cosmic dust that made the Milky Way. The way his brother talked about them made the stars seem like the next best place to the water. But he supposed he should be grateful for the artificial lights now, as they were his only source of light in the night since the moon was behind the clouds.

The tide lowered as the night went on, and Keith followed it with his eyes when he wasn't swimming laps from buoy to buoy. He followed it further down, watched it reveal finer sand underneath. As the water receded, Keith discovered just how far the beach curved down, and how quick the tide revealed it. He’s been there for quite some time, but surely no longer than a few hours.

Keith took in the new view of the beach and his town from the far lower point. The reflection at low tide wasn’t as prominent as the high tide but it was just as beautiful. The cliff, even though the tide only went down about ten feet, it seemed to have grown by twenty or thirty. He blinked a few times and rubbed his drowsiness away before he saw it: a cavern mouth at the water’s edge. 

Keith swam towards it, finding that the water covered the sand that led into it, leaving him to wade through the water at knee-level through the entrance. He pulled his goggles down and pointed his flashlight forwards as he walked.

It took a bit of distance, but Keith climbed uphill and found the back of the cave. The rock of the walls were the same texture as the outside, but darker than the sand under his feet. The contrast was even starker in the moonlight, which flooded into the cave from a tall opening in the rock. 

Upon closer inspection, Keith determined it to be the mouth of the volcano, the full moon almost at the center of it.

Keith’s spine ran cold at the sight. Every nerve screaming at him something was so, incredibly wrong. He knew low tides didn’t appear with the moon in his phase and in that position of the sky, suddenly hyper-aware that he was currently below sea level.

Keith rubbed his eyes to readjust his vision. The moon didn’t change, but the water level did. He could swim well, but only if there wasn’t too strong of a current to fight against. If the tide kept coming in, he wouldn’t be able to get out.

Keith ran for the mouth of the cave. The water passed knee level, growing colder, sending chills up his spine as the cool, salty liquid reached his thighs. His waist. His chest.

He pulled his goggles on, diving in to try to swim out. He could barely make out the entrance with the beam of his flashlight, but it was no use if he was only getting pushed away from it, which is exactly what the current was doing.

With no other choices, Keith ran back. Or rather, he let the water carry him there, as that was the faster option to running through water at this point. He braced his arms against the slick wall, tears falling into the water that already passed his ankles.

Keith couldn’t bear to watch the rising water. He pressed his back against the rigid rocky walls and craned his neck towards the moon, now less than a hair’s width away from the center of the volcano’s mouth. His tears fell harder, and he whispered, pleading, as if the moon couldn’t hear him but was his only salvation.

“Please,” he choked as the water level brushed his hips. “Please, please, please-”

_I don’t want to die._

The words were in the forefront of his mind but were unable to leave his lips, swallowed by the sea as the last part of him was submerged.

The water’s pace quickened, and Keith could only think of one person as his life flashed before his eyes and the water touched the nape of his neck.

“I’m sorry, Shiro,” the bubbles leaving his mouth carried those words to the surface for Keith.

_I should have listened to you._

Keith struggled to keep up with the rising surface, but his body felt like lead, as if gravity was weighing him down against the buoyancy.

He tried to scream, make any noise as loud as he could in the futile hope that someone might hear him. He screamed and screamed until his legs were numb and his insides were on fire.

He screamed until bubbles and moonlight filled his vision.

He screamed until his eyes slipped closed.

* * *

Keith didn’t come downstairs for breakfast. His bike was missing. His bed was empty. Shiro had a strong suspicion about where his little brother was, and hoped to whatever god or gods existed that he was wrong.

He wasn’t.

“KEITH!”

Shiro found him. Unresponsive, wet, cold, but alive. He ran over, dropping to his knees and cradling Keith’s head.

“Damn it! Why didn’t you listen to me?”

Shiro carried Keith out of the cave and up to the gravel trail where Keith left his bike and shoes. Shiro pulled out his phone and called emergency services when Keith was placed on his back

“Hold on, buddy, I’m getting help,” Shiro said to him, desperately hoping Keith could hear him as he shook his shoulder..

The phone line clicked as someone took Shiro’s call.

“What’s your emergency?”

Shiro frantically answered the operators questions as he performed CPR.

* * *

Keith and Shiro walked through the door Sunday morning. Through a series of very fortunate occurrences, Keith was discharged within 24 hours. Keith regained consciousness and his body temperature regulated itself on the ambulance ride. When he was assigned a bed, all his vitals were stable and stayed that way throughout the night. No water was found in his lungs. Whenever he drank water, his bodily systems healed themselves quickly. Shiro was at his side through the whole ordeal and then brought him home. 

The ride back, to Keith’s dismay, was silent and awkward.

“What got into you, Keith?”

He didn’t respond, only slumped back on the couch and averted eye contact.

“Keith!”

“I dunno,” he mumbled.

Shiro sighed. He sat down next to Keith, mindful of how picky Keith could be about personal space.

“Well, if you won’t tell me why you went, do you at least want to talk to me about what happened?” Shiro proposed. “If you don’t want to share, you don’t have to, it’s just-” he let loose a deep exhale. “You really scared me, I thought that-”

“I’m sorry.” It was the first thing Keith said to his brother since the incident that wasn’t a mumble. “Honestly, I don’t remember much, just the tide coming in really fast and...” he paused trying to remember as much as he could. “I was having trouble staying at the surface and something was pulling me underwater. That’s about it. Everything else is blurred out.”

“I have no doubt, it was a traumatic experience,” Shiro answered, putting a reassuring hand on Keith’s shoulder. “Thank you for telling me. And you know you can come talk to me about this again if you want or need to, okay?”

“Yeah.”

Shiro dropped the subject, moving on to what they should get for lunch.

* * *

The entire day, Keith found himself drawn to that secluded beach. Keith thought it was the nagging feeling one gets when they know they've forgotten something but couldn’t recall what. His bike was left at the base of the volcano. Shiro was hesitant to let him go but Keith needed to get it back, so he relented.

Keith looked up when the next lowest tide would be, and went during the day to avoid whatever the hell happened last time he was there. He wasn’t sure why or how he knew this, but the moon definitely had a factor in it. It was the only thing that could possibly explain the strange tidal patterns.

Even with all the forethought, something still itched at Keith. A part of him needed an answer, closure, and only the water could give it to him. Something happened that night but whatever it was, he wasn’t conscious when it happened. He wanted, needed to know. It clawed its way into the forefront of his mind and burrowed itself into his soul.

He looked over the edge where the black sand traveled down to the water’s edge. What he saw couldn’t have possibly have been the lowest tide. The water level was far too high in comparison to that night. At this level, the mouth of the cave would be completely submerged in water.

Keith kicked his shoes and socks off and dropped down to the line where the sea met the sand. He leaned forward as much as he could, but saw no sign of the cave.

He pulled his pant legs up his calves, walking into the water to get a better look.

His legs grew numb and he lost his balance, his body swallowed by the water.

Keith tried to bring his legs under him so he could stand, but they felt as if they were weighted and fused together. He could feel the sand against them, but the texture was much different.

The sand was smooth when he pushed himself forward, into the water, but was coarse in every other direction, digging uncomfortably into and under his skin when he tried to move. Keith wanted to reach back and scratch at the discomfort, but when he did that, what he found definitely wasn’t skin under his fingertips.

Without a doubt, there were scales in place of his legs.

Keith used his upper-body strength to bring himself to the surface as best he could. He forced his eyes open, fully expecting a burning sensation that never came as saltwater trickled into them. Instead, he found his vision was just as clear and painless when he looked through the water over his eyes as it was when seeing above water.

He looked down, a column of shimmering red grew from his middle, stretching to a length twice that of his legs and ending in a large fin -wider than his shoulder span- that matched the smaller ones trailing down his sides.

Scales also appeared in uniform blotches across his chest and stomach. Keith reached around to his back. He felt them trail up from his hips to as far up his back as he could reach. While he couldn’t know for sure, he was almost certain there were more that he couldn’t see that decorated other places of his skin. He was aware of the small ones on his neck and face, but he was blind to most of his backside.

Lastly, he took in the lines of scales that stretched from his upper arms to his fingers, framing the webbing between them and the dark claws that replaced his fingernails. Out of all the changes to his body that he just started to process, this was probably the most unsettling to Keith.

While he certainly wasn’t used to seeing his body like this, it didn’t feel wrong. The scales and fins felt as much a part of him as his hair or his digits. Part of him knew he should be scared out of his mind from the sight of his tail, fins, and claws, but he wasn’t. It felt no different from finding a new freckle or mole on your body that you’ve never noticed before.

Keith battled with his thoughts. One part of him knowing he should have a certain reaction that he wasn’t displaying, and another telling him to stop overthinking things. He wasn’t sure if he even wanted to yell for help, but he knew for a fact that he didn’t want any strangers to see him like this. Out of instinct, he went back underwater. His best hiding place. His second home.

He screamed when he was submerged, allowing himself to let go of the overwhelming indecision he was experiencing. He screamed until he grasped his head with both hands and ran out of air, inhaling water effortlessly to scream again.

Great! 

_Gills._

_I’m a fucking mermaid._

Keith caught his breath, adjusting to the slits on either side of his neck. Breathing through them became an involuntary action when he didn’t think about it. When he did, it was really weird, so he tried to put his mind onto something else.

Breathing in through his mouth or nose didn’t work either, only leaving a burning sensation like having milk fly through one’s sinuses when they laughed while drinking it. Breathing out was a different story. It was much easier and possible, since it was just a matter of releasing air bubbles like someone normally would underwater.

Keith knew kicking like he had two legs wasn’t an option, so he opted for ‘dolphin kick’. He’d never get his old swim lesson terminology out of his head, considering how often it was drilled into his mind- living with Shiro, who works as a children’s swim instructor, definitely contributed to that-. He beat his lower half while envisioning he was kicking both legs in the same direction. His body bolted forward from the action, much further than if it would have been just his feet at the end of his tail.

Keith did it again, not paying attention to the position of his torso. Instead of going forward, he was sent in a sharp diagonal direction, which his body protested against by sending a sharp pain through his side. 

He tried again. He stayed forward this time, and kept going. His speed increased, gradually going faster and faster. He tried to stop himself by quickly surging his tail forward while leaning his torso back, but the action took control of his upper body. Another pulse of pain shot through him, making Keith wince and place a hand over the area that hurt him most.

He tried again. And again. He succeeded each time, but found his middle quickly getting sore from the activity.

Keith broke the surface to test his vision, something he hoped wouldn’t be quite so physically exerting. As he swam, he found that water was as clear to him as if he had his goggles on. Above the surface, his vision was no different from before the incident in the cave. He bobbed his head up and down to compare the difference, only to find there wasn’t one in how well he saw life in and above water.

His hearing in the water was much sharper. He couldn’t communicate any better or worse as far as he could tell, but the swishing of fish as they zipped by and the lapping of the waves as gravity pulled them down was much crisper than it was prior to two nights ago.

Although, if Keith was biased, he would say that nothing beat the sound of crashing waves from above the surface.

In an instant, Keith remembered why he came into the water in the first place. He swam in the direction of the cliff, searching for the cave.

He didn’t find anything. Just sleek rock. No openings, no cracks, nothing.

Keith swam back and forth along the wall. He followed the wall away from the beach until the rock faded into mud and grass. He followed it back with one hand brushing the jagged surface, hoping that if he couldn’t see an opening, he might be lucky enough to feel one.

He didn’t. All he found were some shallow cuts from a few too-sharp edges when he pulled his hand away.

A few small sharks tailed him due to the scent of his blood. They nipped at his fingers, but Keith outswam them easily enough, keeping his injured hand out of the water just to be safe.

Keith managed to swim the remaining distance back to the black-sanded beach without the aid of his hands or arms, which while a good thing to know, was a lot more physically exerting. Keith’s abdomen certainly agreed.

“Keith! Are you okay?” Keith heard the all too familiar voice from the beach.

“Uhm… hi Shiro. Yeah, I’m okay, I think,” Keith softened his voice on the last two words. 

“Why are you in the water? What’s on your face? Are you bleeding?” Shiro called.

“Not blood.” Keith quickly put his hand underwater, not sure how he was going to get out of this one. “What are you doing here?”

“You were supposed to be back home an hour ago. I got worried.”

“Sorry about that.”

“Are you swimming? The water is really dangerous here! There’s jellyfish and sharks everywhere.”

As if on queue, Keith felt a slight tingling sensation on his uninjured palm. A small jellyfish, a definitely poisonous one, bumped his hand repeatedly. Instinctively, Keith petted it back, stroking the rounded top and finding the stingers ticklish against his fine scales and skin.

Apparently being a mermaid makes you tolerant of jellyfish venom.

“They’re not that bad.”

“They’re _not_ -!” Shiro pinched the bridge of his nose. “Answer my question, please.”

“Which one?”

“Why are you in the water?” Shiro repeated.

“I fell in.”

“You did?” the older responded, not sunding convinced.

“Yeah, I… shit,- look, the cave is gone, Shiro! And there’s no way this is the lowest tide of the day. Even if it was here, it would be completely underwater!”

“Good, I don’t want you going back in there.”

“You don’t get it, Shiro! I need-I needed…” Keith groaned in frustration. “Something happened when I was in there, and the cave has to have something as to why.”

“What do you mean ‘why’? You got trapped, almost drowned and passed out.”

“That’s not what I-! I just-! Ugh, just, okay, I’m going to swim closer. Look at me, and see for yourself.”

Shiro obliged. Every step closer to him as Keith swam to shore slowly turned his confused expression into one of shock. The reaction that Keith kicked himself for not having. The reaction he should have had but didn't.

“You…” Shiro tried to gather his words, as if he was worried he'd say the wrong thing. “The cave did that to you?”

“It must have!” Keith was getting impatient now. “I haven’t been in the water since and the first time I go back in _this_ happens.”

“How did it happen? When?”

“I touched the water and after ten, maybe fifteen seconds I have fins and gills.”

A pause passed between them. “Maybe getting you out of the water is what we need to do.”

Keith extended his uninjured hand out, watching Shiro jump slightly at the scales, webbing and claws trailing almost the whole length of his arm. “Well, I’m going to need some help.”

“Yeah,” Shiro nodded and took a step back. “Give me ten minutes, I’m going to get you a bunch of towels from home. Think you can hold out that long?”

Keith looked towards the water, then back to Shiro. “Sure.”

“Alright,” Shiro took a few steps back, clearly still processing his brother’s new form. He finally turned around and got to the car. “Okay, towels.”

“And some scissors!” Keith called after him. “My bike got stuck in some vines!”

Shiro nodded in acknowledgement, revving the car’s engine and driving away.

Keith turned around on his hands and slid back in the water, determined to find that cave again.

* * *

“I feel stupid.”

“You can’t exactly walk, Keith.”

“That’s not what I’m talking about, I mean why do I have to cover my face?”

“This might sound bad, but,” Shiro sighed, “I’m your brother, I care for your safety, and I don’t want the neighbors asking questions.”

Keith pulled at the towel covering his face. “It’ll look weirder if you walk up to the door carrying me all covered up like a dead person.”

“You still have the scales on your face. I’m just looking out for you and I know you don’t like it when strangers or neighbors ask questions.”

“I could just push my face into your shoulder, then they’d still see my hair. They’ll probably still ask questions, but at least they won’t get suspicious.” Questions being asked was going to happen sooner or later, and Keith knew accepting that now was better than fighting it.

Shiro nodded. “That’s... actually a much better idea, good thinking.”

“Thanks.”

They made it back to the house. The plan all set, executed almost flawlessly, and almost failing when Shiro nearly tripped under Keith's weight in his arms. Not accounting for the extra weight due to the tail didn’t cross either of their minds. Keith practically glided across the sand at the beach and was hardly short of flung into the passenger's seat of the old pick-up truck Shiro drove. Most of their concern went into keeping all of Keith’s scales, claws and gills concealed in addition to the tail. The ten meter path felt closer to one hundred and it was no wonder, considering how fast Keith's mind was firing off with 'what if' scenarios, and the discomfort that wracked his body from tightly folding his tail up into a position it probably wasn't meant to be in. When they were inside, Keith released the breath he didn't know he held. Shiro plopped his brother on the couch and grabbed more clean towels before throwing them at his brother.

“Hey!”

“Dry off. I have a theory.”

“Okay! Okay,” Keith pulled off the wet towels and made quick work in drying himself off with a fresh one. Contorting himself to reach the lower half of his tail was very uncomfortable. Shiro grabbed an old hair dryer from the bathroom and began blow-drying Keith's hair.

“Do you have to do that?”

“If every part of you is dry, you might get your legs back.”

“ _Might_?”

“Everything’s speculation here, Keith, but we do know that something happened to you two nights ago. You were soaking wet when I found you, and you didn’t have a tail. You haven’t gotten any water on you since and the minute you touch it, or at least when you touch sea water, you sprout a tail and well,” Shiro gestured in the general direction of Keith’s upper body with the dryer in hand, “other things.”

“Haven’t noticed,” Keith deadpanned. “Are you done with my hair yet?”

Shiro ran a hand through Keith’s hair. “It’s dry, but-” as the words left his mouth, the dowels that were draped over the lower half of Keith’s tail dropped to the carpeted floor with a muffled thud. Keith pushed aside the other towels covering him to find his jeans and two skinny legs. Keith and Shiro laughed with relief.

“Guess I just gotta be dry to have my legs back,” Keith exhaled. “Was that your theory?”

“Yeah, that was my theory.” Shiro began picking up the towels to throw them in the washing machine. “You know you can walk now, I’d like some help.”

“Can’t touch water,” Keith reiterated, leaning back against the couch and resting his elbows on the edge. He was already smiling about all the chores he’d be able to get out of. Mopping, laundry, washing dishes, bathing the future puppy that Shiro promised they would adopt-

“Then grab the unused ones and put them away, or...” Shiro turned to face his brother, a small smirk on his face, “you know, rubber gloves are also a thing. You'll find some under the kitchen sink.”

Keith groaned in frustration. For what it was worth, those few seconds of being liberated from the better half of his chores was nice.

**Author's Note:**

> I've been into VLD for a while, but this is the first fic I've written for the show. I'm worried about keeping everyone in character and staying true to the dynamics between everyone so if something is off, I apologize.
> 
> Comments, questions or critique? I'd love to hear it down in the comments below!


End file.
